A door is a movable structure used to open and close off an entrance, typically consisting of a panel that swings on hinges or that slides or rotates inside a space. When open, doors admit ventilation and light. A door may be used to control physical atmosphere within a space so that an interior may be more effectively heated or cooled. A door may also be significant in preventing the spread of fire. Doors also act as a barrier to noise, inclement weather, and physical intrusion into a specifically defined space.
In contemporary motor vehicles, besides carrying the above-mentioned functions, doors are frequently reinforced to reduce the degree of damage to the door structure in cases of impact between the vehicle and another object. Such reinforcements may be designed to allow the door to withstand some level of impact from another object without permitting intrusion into the vehicle's passenger compartment or sustaining extensive damage to the vehicle's safety systems. Additionally, such reinforcements may be tied to the door structure to allow the door to absorb a portion of impact energy and, for occupant protection, route the energy away from the vehicle's passenger compartment.